First off, Command & Conquer 4: Tiberian Twilight
is always online. Even when playing through the single-player campaign, you have to sign on to the game's online portal. In other words, if you can't get online with the game, you can't play. It's a cruddy trend in PC games that in all likelihood is here to stay and as much as I don't like it, at least there's an attempt to have it make sense in the grander scheme of things. The series has always done a great job of creating a "you're there" atmosphere, so jumping into what looks like a high-power military network is awesome, especially when you can easily communicate with members of the in-game community. At the same time, it's a bit unreasonable to expect every player to be able to get online at all times. I get the need for strong DRM, but this might be a little too much.
After a short tutorial, Tiberian Twilight gives you the opportunity to follow Kane and join NOD or join GDI. As with other games, you can go back and play through the opposing campaign and get a different feel for the story by playing through the same mission, only under different circumstances. Compared to past games, missions are much smaller in scope and design.
Rather than offering a massive battlefield and opportunity to build a sprawling base, everything is limited to a mobile base (called Crawlers). Crawlers are limited to one at a time and can be one of three combat roles: Offense, Defense or Support. Each type can create different units and if you want to switch types, you first need to scrap your current one. During single-player missions, this isn't much of an issue; you can usually get by with your original choice. In multiplayer, it places a lot of emphasis on team strategy and makes better sense.
Multiplayer is Tiberian Twilight's element and one of the few times the gameplay feels right. Combat roles are more intriguing and make more sense when playing with (and against) human players. There's more of a chess game going on between players. Multiplayer is limited to one mode, Domination, which plays a lot like the command point control modes found in shooters. Keeping your units near a command node generates "points" until one team reaches a set score. Matches are incredibly dynamic (when played right, that is) and it isn't uncommon to scrap your Crawler mid-game in response to what's going on.